Okay so my son and I are well into Season 4 of TCW (holy crap those Citadel and Umbaran/General Krell episodes, so awesome!) but I wanted to take a moment to discuss the Mortis episodes from Season 3. First off I am very glad that that "planet" was not featured in Episode 9. For one thing in my mind it isn't even a real place but is instead a strange convergence of the Force in deep space that gave Anakin, Obi-Wan, and Ahsoka a shared vision. A vision of future events and themes.
One future event in particular being that of life being passed through a Skywalker and into the dead body of young female Jedi that he cared for, resurrecting her.
The mosaics on the ground on Mortis were also quite similar to the mosaics in the sanctuaries of Ahch-To. Not to mention the thematic similarities between the Father, son and daughter with ST Luke, Kylo, and Rey. A quick Google search shows that others picked up on that before me:
Cheating death, never fulfilled until the ST, the visions of Mortis, never fulfilled until the ST. I'll probably forever go back and forth on whether the PT/TCW is canon or not canon but I must admit that I quite like how it all ties together in the end.